page_title: Lord Howe Island
section_title: Basalts and calcarenite
hierarchical_section_title: Lord Howe Island / Geography / Basalts and calcarenite
caption_attribution_description: English: Basalt dykes, Lord Howe Island
context_page_description: Lord Howe Island is an irregularly crescent-shaped volcanic remnant in the Tasman Sea between Australia and New Zealand, 600 km directly east of mainland Port Macquarie, 780 km northeast of Sydney, and about 900 km southwest of Norfolk Island. It is about 10 km long and between 0.3 and 2.0 km wide with an area of 14.55 km², though just 3.98 km² of that comprise the low-lying developed part of the island.
  Along the west coast is a sandy semi-enclosed sheltered coral reef lagoon. Most of the population lives in the north, while the south is dominated by forested hills rising to the highest point on the island, Mount Gower. The Lord Howe Island Group comprises 28 islands, islets, and rocks. Apart from Lord Howe Island itself, the most notable of these is the volcanic and uninhabited Ball's Pyramid about 23 km to the southeast of Howe. To the North lies a cluster of seven small uninhabited islands called the Admiralty Group.
  The first reported sighting of Lord Howe Island took place on 17 February 1788, when Lieutenant Henry Lidgbird Ball, commander of the Armed Tender HMS Supply, was en route from Botany Bay to found a penal settlement on Norfolk Island.
context_section_description: Two periods of volcanic activity produced the major features of the island. The first about 6.9 million years (Mya) ago produced the northern and central hills, while the younger and highly eroded Mount Gower and Mount Lidgbird were produced about 6.3 Mya by successive basalt (an extrusive igneous rock) lava flows that once filled a large volcanic caldera (crater) and can now be seen as horizontal basalt strata on mountain cliffs (at Malabar and Mount Gower) occasionally interspersed with dikes (vertical lava intrusions). Geological pyroclastic remnants of volcanic eruption can be seen on the 15 ha (37-acre) Roach Island (where the oldest rocks occur) and Boat Harbour as tuff (ash), breccia (with angular blocks), and agglomerate (rounded 'bombs'). Offshore on the Lord Howe Rise, water depths reach 2,000 m (6,600 ft) falling to 4,000 m (13,000 ft) to the west of the rise. From the dimensions of the rock on which the island stands,  the island has been calculated to erode to 1/40th of its original size.
  Rocks and land at the foot of these mountains is calcarenite, a coral sand, blown inland during the Pleistocene between 130,000 and 20,000 years ago and cemented into stratified layers by water percolation. In this rock are fossils of bird bones and eggs, land and marine snails, and the extinct endemic horned turtle (Meiolania platyceps) now thought to be an ancient relictual nonswimming tortoise with relatives in South America. The crescent of the island protects a coral reef] and lagoon, the barrier reef, at 31°S, is the most southerly in the world. Beach sands, rather than consisting of quartz grains derived from granite, as on the mainland, are made of fragments of shell, coral, and coralline algae, together with basalt grains, and basaltic minerals such as black diopside and green olivine. The lowland consists of alluvial soils.
  The island continues to erode rapidly and is expected to be fully submerged within 200,000 years, taking an appearance akin to the Middleton and Elizabeth Reefs.
caption: Basalt dikes on the eastern cliffs
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